1. Field of the Invention
The present invention relates to a manual chain block which is provided with a drive member having a hand wheel and a mechanical brake and is adapted to drive a load sheave through the mechanical brake by an operation of the hand wheel.
2. Description of Prior Art
In a conventional manual chain block, for example as disclosed in the Japanese Utility Model Publication No. Sho. 62 (1987)-16477, a load sheave is rotatably supported between a pair of side plates, a drive shaft is inserted into a central shaft portion of the load sheave, a hand wheel is threadably secured to an axial one side portion of the drive shaft, a mechanical brake is interposed between the hand wheel and the drive shaft and a reduction gear mechanism is disposed in the axial other side portion of the drive shaft, such that when the hand wheel is operated, the drive shaft is driven through the mechanical brake and the load sheave is driven through the reduction gear mechanism. Further, a hook is disposed between the side plates, a wheel cover for covering the mechanical brake and the hand wheel is attached to one of the side plates and a gear cover for covering the reduction gear mechanism is attached to the other side plate.
The manual chain block having the above-mentioned structure is used not only inside of a building such as a factory but also outdoors, for example at a building site. Though the manual chain block is provided with the wheel cover for covering the mechanical brake and the hand wheel, a chain introducing opening through which a hand chain looped around the hand wheel passes is opened in a side wall of the wheel cover.
Accordingly, when the manual chain block is used outdoors, rainwater enters an interior of the wheel cover through the chain introducing opening during a rainy weather working. Further, when the manual chain block is used at a fine weather day or inside of the building, especially within a dusty atmosphere, dust such as fine sand grains enter the wheel cover through the chain introducing opening.
When the rainwater and the dust enter the wheel cover as mentioned above, they come to be interposed between lining plates constituting the mechanical brake, namely a lining plate interposed between the hand wheel and a brake ratchet wheel and a lining plate interposed between the brake ratchet wheel and a driven member, and respective braking surfaces of the hand wheel, the ratchet wheel and the driven member to which those lining plates are opposed, so that comes out such a problem that a braking performance and a brake releasing performance of the mechanical brake change due to that interposition.
Also has been proposed such a chain block that has the above-mentioned structure as well as an overload preventive mechanism disposed in the drive shaft on its hand wheel side.
This overload preventive mechanism comprises a lining plate interposed between a drive member and a hand wheel on the drive shaft, a resilient member and a load setting and adjusting member and serves to set a slipping load of the hand wheel relative to the drive member by adjusting a pushing force of the resilient member relative to the lining plate by turning the load set ting and adjusting member.
At the time of a load lifting working, when the hand wheel is normally driven by pulling an endless hand chain looped around the hand wheel, the drive member is threadably advanced relative to the drive shaft and then the drive shaft is driven through a transmission mechanism provided with the mechanical brake. As the drive shaft is driven in that way, the load sheave is rotated through the reduction gear mechanism so that a load can be lifted by a load chain looped around the load sheave.
During the load lifting working, when a load larger than the slipping load set by the load setting and adjusting member of the overload preventive mechanism acts on a load side, namely a load chain side, the hand wheel slips relative to the drive member, so that the load lifting after that can be stopped and a level of the lifted load can be maintained by an action of the mechanical brake.
Further, at the time of lowering the lifted load, when the hand wheel is reversely rotated by operating the hand chain, the drive member is threadably retreated relative to the drive shaft and then the load sheave is reversed by repetitions of an action and an inaction of the mechanical brake so that the load lowering can be carried out gradually.
Well, also in the above conventional structure, the overload preventive mechanism and the transmission mechanism are protected by a wheel cover disposed around the outer periphery of the hand wheel. But, since the chain introducing opening is formed in the wheel cover, dust tends to enter an interior of the wheel cover through the chain introducing opening and, at the time of outdoor use, not only the dust but also the rainwater enter through the chain introducing opening, so that these dust and rainwater and the likes happen to enter between respective component members of the overload preventive mechanism and the transmission mechanism.
Then, when the dust and the rainwater enter the overload preventive mechanism, a coefficient of friction and the like of the lining plate of the overload preventive mechanism change and then the slipping load set by the load setting adjusting member changes at its convenience. For example, when the dust enters, the coefficient of friction of the lining plate becomes larger. Accordingly, even though a load larger than the slipping load set by the load setting and adjusting member acts on the load sheave, the hand wheel doesn't slip relative to the drive member to further continue the load lifting working. When the rainwater enters, the coefficient of friction of the lining plate becomes so smaller that the hand wheel starts to slip relative to the drive member at a smaller value than the slipping load set by the load setting and adjusting member to stop the load lifting working.